1. Field of the Invention
The invention relates to optical waveguide devices produced by periodic variation of refractive index in glass substrates including glass optical fibers. More particularly the present invention provides a method employing exposure to energy from pulsed lasers, emitting femtosecond pulses in the near infrared region, to increase the sensitivity of glass to refractive index modifying laser energy emitted in the ultraviolet region of the spectrum to produce optical waveguide devices, also referred to herein as Bragg gratings.
2. Description of the Related Art
Throughout the centuries glass has been used in a variety of scientific and domestic applications to advance the capabilities of modern society. From the early use of prismatic glass for separating light into its component colors, glass has been widely used in optical devices that control or adjust the properties of light beams. A recent and rapidly expanding application of the light modifying properties of glass structures involves the drawing of fine filaments of highly purified glass, more commonly referred to as optical fibers, that direct light signals between light transmitting and receiving locations.
During the late 1970s, utilities began using optical fiber installations for internal communication, and by the early 1980s, a number of small optical fiber networks were installed. The use of such networks has grown to replace previously existing coaxial cable systems. Advantages provided by optical fiber communications networks include lower cost, the use of fewer signal repeaters to correct for signal distortion, and a higher signal carrying capacity than coaxial cable networks.
Special features may be built into selected lengths of optical fibers to be spliced into fiber optic networks. A fiber Bragg grating represents a light-modifying feature that may be introduced or written into an optical fiber by exposure to ultraviolet light. The process of introducing special features such as Bragg gratings into an optical fiber may include a number of steps requiring handling of relatively short lengths of optical fiber during a series of manufacturing operations. An optical fiber typically requires removal of protective coatings before changing the physical characteristics of the fiber to include a Bragg grating. After writing a Bragg grating, the fiber may be annealed and recoated to protect the optical fiber and its imprinted waveguide from physical damage or attack by environmental contaminants.
The magnitude of change in the refractive index of glass substrates, exposed to ultraviolet laser energy, depends upon the sensitivity of the glass to the ultraviolet radiation. It is known that silica glass substrates, such as optical fibers, may be treated to include chemical elements such as germanium in their structure. This produces germano-silicate glasses, which have increased sensitivity to ultraviolet radiation. Further increases in sensitivity to ultraviolet radiation have been achieved using a process known as hydrogen loading. The process of hydrogen loading involves treating glass in an environment of hydrogen at high pressures and elevated temperatures that promote hydrogen diffusion into the glass. Hydrogen-loaded germano-silicate fibers represent the current substrate of choice for manufacture of glass fiber-containing refractive index gratings resulting from exposure to ultraviolet radiation. Such gratings exhibit a marked variation in the magnitude of periodic refractive index change (An) along the length of a grating. The magnitude of refractive index change is known to diminish during the process of grating stabilization, involving high temperature annealing. It is believed that annealing causes some loss of hydrogen and other species that affect the magnitude of refractive index change.
The periodicity of refractive index change varies depending upon the wavelength of radiation and the dimensions of the beam impinging on a glass substrate to change its refractive index. Preceding description of the use of ultraviolet radiation relates to the production of refractive index or Bragg gratings having wavelength characteristics required for fiber optic communications networks. Other types of radiation may cause changes in glass to provide gratings having different periodic spacing of the refractive index variation from those produced using ultraviolet radiation. An example of this is the known use of lasers emitting pulses of energy at femtosecond pulse widths in the near infrared (NIR) region of the spectrum to produce permanent refractive index changes in various glasses.
Devices such as waveguides, couplers and diffraction gratings have been formed by refractive index modification of glass. U.S. Pat. No. 5,978,538 describes a femtosecond laser process for forming optical waveguides in the interior of oxide, halide, and chalcogenide glasses. According to the description of published application WO 01/09899, a femtosecond laser process effects direct writing of light guiding structures into the bulk of soft, silica-based glasses. Other references to refractive index change in glass by exposure to femtosecond laser pulses exists in descriptions by Kondo et al in Optics Letters, Vol. 24, page 646, 1999 and Fertein et al in Applied Optics, Vol. 40, page 3506, 2001.
Refractive index changes in glass result from multi-photon photochemical reactions during exposure to femtosecond laser pulses. Exposure to short femtosecond energy pulses in this way yields waveguides that, unlike hydrogen-loaded glasses, retain a relatively stable change in the magnitude of refractive index variation. Although changes in refractive index remain relatively stable, visible defects accompany waveguide formation in several glasses including silica and germano-silicate glasses. The defects compromise the mechanical strength of optical fibers after fabrication of optical devices such as optical fiber waveguides. This suggests that near infrared laser pulses, operating at femtosecond pulse widths, are unsuitable for use with optical fibers, due to the occurrence of damage to the processed glass. A further disadvantage of near infrared femtosecond laser pulses is the minimum diffraction-limited spot size obtainable by focusing lasers of this type. The minimum spot sizes achievable after laser beam focusing, in this case, is approximately 2 μm-3 μm. Bragg gratings having suitable wavelength characteristics for telecommunications applications require focusing of the exposing laser beam to produce fringe periodicity of approximately 0.5 μm.
With increased growth of fiber optic telecommunications networks, there will be increasing demand for optical fiber devices including modified glass structures, such as Bragg gratings. In anticipation of this demand there is a need for a process to produce devices, free from mechanical defects, that remain stable after formation.